The goal of this project is the understanding of the structure and function of endogenous and recombinant murine retroviruses expressed by leukemia cells induced by inoculation of mice with Friend murine leukemia virus. To identify proteins of these viruses, we have derived a panel of monoclonal antibodies which appear to recognize the endogenous portions of the envelope proteins of recombinant viruses. These antibodies have been used in a rapid test to identify different groups of recombinant viruses and to distinguish these from non-recombinant viruses. Furthermore, some of these antibodies recognize cell-surface molecules of normal uninfected fetal hematopietic cells. The possible roles of these endogenous gene sequences in differentiation and leukemogenesis are being studied.